1. Field of the Invention
The present invention relates to a novel electrolytic gold plating method in which a degree of deterioration of a gold plating solution of gold sulfite complex is measured and monitored, and an apparatus for the electrolytic gold plating method.
2. Prior Art
As a conventional electrolytic gold plating method known is the gold cyanide plating method in which gold cyanide complex is used as a major component of the plating solution. Because the gold cyanide plating solution is very stable, abnormal deposition of metallic gold does not occur. However, since cyanic compounds have strong toxicity and accordingly have a problem in the environmental aspect, non-cyanic group plating methods have been developed and practically used.
In recent years, a gold sulfite plating solution having a major component of gold sulfite complex is growing widely used. As described above, the pollution-free electrolytic gold plating has been made possible since the gold sulfite plating solution does not have toxicity and the environmental effects are sufficiently considered. However, although the environmental effects of the gold sulfite plating solution are sufficiently considered, the stability of the solution is not sufficiently stable and accordingly the abnormal deposition of metallic gold is apt to occur during using. The reason exists in the instability of gold sulfite complex. That is, the stability of gold sulfite complex is very small compared to the stability of gold cyanide complex. Gold sulfite complex is deteriorated and decomposed to form univalent free gold ions, and the gold ions form metallic gold through disproportionation reaction. The metallic gold is in a form of very small particles in the initial stage, but the gold particles are growing by coagulation of the particles. Then, metallic gold is abnormally deposited on the surfaces of the components of the apparatus inside the plating bath irrespectively of current-carrying. This phenomenon causes a problem of incapability of normal gold plating. A gold plating solution using gold sulfite complex is disclosed in Japanese Patent Application Laid-Open No. 9-59792, Japanese Patent Application Laid-Open No. 10-251887 and Japanese Patent Application Laid-Open No. 11-61480.
When the gold abnormal deposition described above occurs, the solution must be drained from the plating apparatus to clean the inside of the apparatus. However, gold is a very stable metal, and accordingly, can not be dissolved and removed using an agent such as a common acid. Therefore, the plating apparatus needs to be recovered to the initial condition by performing cleaning. Therefore, this requires a large amount of time and cost, and is extremely uneconomical.
Further, in the above-described gazettes, there is no description on the deterioration of the gold sulfite complex plating solution when gold plating is performed using the solution.
An object of the present invention is to provide an electrolytic gold plating method which can always stably perform electrolytic gold plating and can prevent incapability of performing electrolytic gold plating by continuously or intermittently detecting a deterioration state of the gold sulfite complex plating solution while the gold plating is being performed using the gold sulfite complex plating solution, and to provide the electrolytic gold plating apparatus.
The present invention is characterized by an electrolytic gold plating method for performing electrolytic gold plating on a surface of a substrate body using a gold sulfite plating solution, the method comprising the steps of detecting a deterioration degree of the plating solution; and performing the plating. It is preferable that the detection of a deterioration degree of the plating solution is performed before starting plating or always or intermittently performed during plating.
In regard to the detection of a deterioration degree, it is preferable to irradiate light to the plating solution and then measure a light intensity of the specific absorption wavelength, preferably, a light intensity of the specific absorption wavelength of 310 nm. It is preferable to perform at least one of detection of an amount of gold colloid formed in the plating solution, measurement of a pH of the plating solution, measurement of sulfurous acid in gold sulfite complex in the plating solution, and measurement of a concentration of sulfuric acid in the plating solution.
Further, the present invention is characterized by an electrolytic gold plating method comprising the step of performing at least one of addition of the plating solution, adjustment of pH, adjustment of sulfurous acid concentration and adjustment of sulfuric acid based on the value obtained by measuring at least one of an amount of gold colloid of said plating solution, a value of pH of the plating solution, a concentration of sulfurous acid in gold sulfite complex in the plating solution and a concentration of sulfuric acid in the plating solution.
The present invention is characterized by an electrolytic gold plating apparatus for performing electrolytic gold plating on a surface of a substrate body using a gold sulfite plating solution, which comprises a detecting means for detecting a deterioration degree of the plating solution, and is also characterized by an electrolytic gold plating apparatus which comprises a monitoring unit for displaying the degree of deterioration. It is preferable that the detection of a deterioration degree of the plating solution is performed before starting plating or always or intermittently performed during plating.
It is preferable that the detecting means comprises at least one of means for irradiating light to the plating solution and measuring a light intensity after the irradiation, means for measuring a pH of the plating solution, means for measuring sulfurous acid in gold sulfite complex of the plating solution, and means for measuring sulfuric acid of the plating solution. It is preferable that the means for measuring the light intensity is an absorptiometer, and that the means for measuring the pH is a pH meter using a glass electrode, and that the means for measuring the sulfurous acid in the complex is an automatic titrator or a liquid chromatograph.
That is, the inventors of the present invention conducted detailed study on gold sulfite plating solution, and found that the absorption characteristic of the plating solution, the pH of the plating solution, the concentration of sulfurous acid and the concentration of sulfuric acid in the plating solution were changed as deterioration of the solution progressed. As deterioration of the solution progressed, the absorption intensity of the specific wavelength of the plating solution is increased, the pH and the concentration of sulfurous acid in the complex are decreased, and the concentration of sulfuric acid is increased. Therefore, these are detected to be used as degrees of the deterioration of the plating solution.
Further, the present invention makes it capable to stably perform gold plating by detecting and analyzing one kind or combination of more kinds of the above-described factors to monitor the deterioration state of the gold sulfite plating solution. It is preferable that the abnormal deposition of gold is predicted to output an alarm.
Furthermore, the present invention is characterized by an electrolytic gold plating apparatus comprising an automatic adding solution supply unit for adding the plating solution based on a value obtained by measuring at least one of an amount of gold colloid of the plating solution, a value of pH of the plating solution, a concentration of sulfurous acid in gold sulfite complex in the plating solution and a concentration of sulfuric acid of the plating solution; an automatic pH adjustment unit for adjusting pH; and an automatic water supply unit for supplying water for evaporated water.
The present invention is effective for the electrolytic gold plating to form lead terminals and wires used in a semiconductor device.